The long-term objective of our research is to understand how the immune response to the antigens of a primary autochthonous tumor influences the growth of that tumor. We are interested both in influences which operate before the emergence of grossly detectable tumor and those which operate beyond this stage. The former may affect the oncogenic insult itself, premalignant stages of the neoplasm or small numbers of malignant cells not yet grossly detectable. The latter must necessarily affect the growth or regression of an established tumor. The system chosen for this study is that of primary murine Rous sarcoma. To date, we have shown that inoculation of normal adult syngeneic lymphoid cells early in the latent period of oncogenesis, increases the probability of overt tumor development, that this effect correlates with a delay in the ontogeny of cellular immune responsiveness and that it is mediated by non-T cells.